Hydride gases, especially ammonia, are used in a number of processes including the manufacturing of semi-conductors and LEDs. Other hydride gases, such as arsine (AsH3) and phosphine (PH3), are also used to manufacture semiconductor thin films such as gallium arsenide (GaAs) and gallium phosphide (GaP), which are used in high speed data transmission equipment, cell phones, videophones and commercial satellites. Additional hydride gases used in the manufacture of semiconductors include diborane (B2H6), disilane (Si2H6), germane (GeH4) and silane (SiH4). For purposes of the invention, hydrogen (H2) is also considered to be a hydride gas.
Advances in integrated circuit semiconductor technology have included the development of semiconductors with copper interconnects instead of aluminum interconnects. Copper interconnects are advantageous in that copper has less resistance than aluminum, which leads to higher performance in microprocessors, microcontrollers and random access memories.
However, copper tends to migrate over a period of time, so it is necessary to construct barrier layers in the semiconductor to prevent the migration of copper. Such barrier layers are typically made of nitrides such as tantalum nitride (TaN), titanium nitride (TiN) or silicon nitride (Si3N4). These layers are commonly formed by deposition from a hydride gas, such as ammonia (NH3) and/or silane gas.
Further, ammonia is widely used as a source of nitrogen for film development in some thin film applications. The ammonia allows for lower temperature film growth in chemical vapor deposition (CVD) processes. Silane and germane are used for SiGe films and high speed integrated circuits.
In addition, there is current research to develop high performance light emitting diodes (LEDs). These are made by metal organic chemical vapor deposition (MOCVD). Trialkyl gallium reacts with a gaseous mixture of ammonia and hydrogen to be deposited as gallium nitride.
Gallium nitride is also being considered for “blue lasers,” i.e., lasers that emit blue light. Blue lasers are anticipated to be capable of forming compact disks that will have a much higher information density than is presently possible with compact disks produced with red laser light. Gallium nitride for such blue lasers would be manufactured from the same type of ammonia/hydrogen gaseous environment as described above for the high capacity LEDs.
The LEDs, blue lasers and integrated circuits are all manufactured with electron accepting p-type dopants. Such products are extremely sensitive to the presence of electron-donating n-type materials, and very small concentrations of such n-type material are sufficient to deactivate the p-type dopants and impair or destroy the performance and operability of the integrated circuits, LEDs and blue lasers. Oxygen is a particularly efficient n-type material, and the presence of molecular oxygen is detrimental to the desired band gap properties in the semiconductor material. Even very low concentrations of oxygen (<10 ppb) can be sufficient to cause a significant reduction in performance or operability so as to require discarding the product after manufacture. The presence of oxygen in the manufacture of LEDs results in a decrease in the photoluminescence intensity.
Water vapor and carbon dioxide gas are two other detrimental contaminants in hydride gases, which lead to degradation of products that are formed by deposition of active layers of metals or metal compounds from a hydride gas environment. Water is one of the most common and yet most difficult impurities to remove from the gases. Water is ubiquitous in almost all ambient environments. Even systems that are nominally referred to as “dry” usually have significant amounts of water, and most drying processes can reduce the moisture content of a gas only to a “minimum” that is still in the parts per million (ppm) range. However, because water contents in the ppm range are quite acceptable for many purposes, there are numerous patents and articles in the literature dealing with such types of “ppm drying processes.”
In the manufacture of the aforesaid LED, blue laser and integrated circuit products, however, moisture contents of the depositing gases which are in the ppm range are excessively wet. Impurities in the gas result in the loss of photoluminescence intensity when water is incorporated as a dopant contaminant in gallium nitride films. To form satisfactory products, the water content of the depositing gases must be reduced to the parts per billion (ppb) range, usually to no more than about 100 ppb. See Whitlock et al., “High Purity Gases,” in Ruthven, ed., ENCYCLOPEDIA OF SEPARATION TECHNOLOGY, vol. 1, pp. 987-1000 (1997).
Attempts to use materials such as reduced nickel or copper catalysts to remove contaminants such as oxygen, carbon dioxide and water from hydride gases have not been successful. While contaminant removal can be effected for short periods of time down to the 10 ppb level, the reactive effects of the hydride gases, especially ammonia, very quickly cause the materials to degrade and contaminate the gas stream with metal complexes. Although pre-existing impurities may be reduced with these materials, the introduction of new impurities to the manufacturing process is unacceptable.
Some prior art processes have been described in which oxygen has been removed from ammonia streams by metals or metal complexes serving as “getters.” However, these have been relatively ineffective at sufficiently decontaminating a gas stream. In addition, the getters are typically deposited on substrates, such as silica or zeolites, which do not play a central role in the decontamination process, and may themselves be degraded by the hydride gases. See, for instance, U.S. Pat. No. 5,496,778 (Hoffman et al.), U.S. Pat. No. 5,716,588 (Vergani et al.) and U.S. Pat. No. 4,976,944 (Pacaud et al.); PCT Publication No. WO 97/06104 (SAES Getters S.p.A.); and European Patent No. EP 0 784 595 B1 (SAES Getters S.p.A.).
Alvarez (U.S. Pat. No. 6,241,955) teaches the use of mixed compositions of high surface area (>100 m2/g, preferably 200-800 m2/g) reduced metal oxides for the removal of a number of contaminants from hydride gases including oxygen, carbon dioxide and water vapor from hydride gases, including low alkyl analogs of hydride gases. The metal oxides are preferably those of manganese or molybdenum and can be optionally mixed with iron oxide. Such materials were found to be stable for long term use; however, water removal efficiencies and water and oxygen capacities were found to be limited such that it is necessary to augment these materials with additional drying agents such as molecular sieves (e.g., Vergani, U.S. Pat. No. 5,716,588).
The problem of removal of contaminant levels down to ≦100 ppb in hydride gases remains a significant problem in the field of production of high purity LEDs, blue lasers, semiconductors, and the like. Those processes that are being used are expensive because of the very short service life of the decontaminating materials and their need for frequent replacement. In addition, since it is difficult to determine the exact rate of deterioration of the decontaminating materials in the presence of the reactive hydride gases, users of such decontaminating materials must schedule their discard and replacement at intervals less than the shortest expected service life. To do otherwise would risk failure of a decontamination unit thereby resulting in a contaminated product. Consequently, current systems require that many if not most of the decontamination units must be discarded while they still have some degree of useful service life left, thus further increasing the expense of the system operations.